The present invention generally relates to strand pelletizing cutters and, more particularly, to a manual strand pelletizing cutter use in testing operations to simulate automatic circular cutters.
Manufacturers of plastic products generally obtain plastic material for the products in the form of small plastic pellets. These pellets are fed into extruders where they are remelted, compressed and forced under high pressure into molds or through die openings to form the final products. It is important that these pellets be as uniform as possible to avoid structural variations in the product. This method of manufacturing is quite different from those industries that create the finished product by employing a wide variety of cutting and forming tools and where the principles of their design, manufacture and application are well understood.
Special cutting tools may be employed when producing materials in the form of small pellets. This is generally known as a strand pelletizing process. Many producers of pellets have failed to adopt strand pelletizing as a viable method or prematurely abandoned it in favor of another procedure due to the lack of understanding and misapplication of the cutting tools required for the last stage of the process i.e., cutting the pellets.
For a better understanding of the automatic cutting of plastic strands into pellets, refer to FIG. 1. FIG. 1 illustrates the material to be cut and the cutting components. The molten material is forced through a heated high pressure pipe line 2 to terminate at a flared nozzle 3. The nozzle 3 is capped by a plate (not shown) which allows the material to escape through a series of small holes or orifices. The orifices may be of any profile but are usually round.
The material is forced through the orifices to form strands 4 which are normally sticky when hot and might weld together if allowed to touch one another. Thus, the orifices are spaced accordingly such that the hot molten material strands 4 do not stick to one another. The sticky characteristics gradually dissipate as the material cools.
The strands 4 then descend through a trough (not shown) filled with water where the strands solidify. Guided by a series of grooved rollers (not shown), the strands emerge from the cooling bath and are captured between a feed roll 6 and a pinch roll 7. In FIG. 1, the feed roll 6 and pinch roll 7 are partially cut away to better allow an unobstructed view of the cutter 8 and bed knife 9. The feed roll continuously feeds the strands into the cutter 8 where they are severed into pellets 10 between the cutter teeth 11 and the bed knife 9.
The feed roll 6 rotates at an adjustably timed relation to the cutter 8 so as to regulate the length of the pellets 10. The force behind the molten material is also balanced with the demands of the feed roll 6 to assure that the cylindrical pellets are maintained as near as possible to the desired cross-section.
During the cutting stroke, a curve 12 is created in the strand. This curve 12 is crucial in the designing of the cutter and cutter blades. Since generally, the curve 12 is not present in a manual testing apparatus, guesses must be made as to what actually happens during cutting.
For clarity, a better explanation will be given in view of FIG. 2. FIG. 2 illustrates a cross-section view through the cutter 8. Referring to the cutting teeth 11, a relief facet or relief land 13 is formed on the back of each tooth. The land 13 or relief extends from the cutting edge 14 to the heel 15 of the tooth. The width of the land 13 surface is critical to the success of the operation of the cutter. If the land is too narrow, the forces encountered by the cutting edge 14 must be resisted by the flank angle 16 instead of the relief angle 17 causing premature breakdown of the cutting edge 14.
If the land 13 is wider than necessary to support the cutting edge 14 certain undesirable reactions may occur. The strand curve 12 begins at the point where the strand 4 leaves the feed roll 6 and continues to the point where the strand 4 is engaged by the cutter 8. The strand 4 is constantly being forced into the cutter 8 by the feed roll 6. When the strand 4 is first captured between the cutting edge 14 of a cutter tooth 11 and the bed knife 9, its forward progress is interrupted while the feed continues, as seen in FIG. 2. At this point in time, the strand curvature begins to develop and continues to increase until the strand 4 is released by the heel 15 of the cutter tooth 11 and is freed to enter the gullet 18. The period of time during which the forward progress of the strand 4 is interrupted is determined by the diameter of the strand 4, the length of the pellet and the width of the land 13. The longer the impedance, the higher arch of curvature. In FIG. 2, the arch of the strand will continue to increase until the heel 15 has passed the cutting edge of the bed knife 9.
While the forward progress of the strand 4 is interrupted, it is storing energy supplied by the feed roll 6. When the strand 4 is finally released by the heel 15, the stored energy is suddenly released and the strand 4 can fly out of control and miss its engagement with the next tooth 11 resulting in non-uniform pellets. It should also be noted that the curve may be horizontal instead of vertical, as shown in FIG. 2, in which case, the energy released by the strand may cause it to move laterally and interfere with neighboring strands with equal undesirable results. In cases where the product material is brittle and easily fractured, too much curve or arch in the strand 4 may cause it to break at the crown of the arch resulting in long pieces of the strand, called longs, appearing in the discharge.
It is possible that certain brittle materials may have failed to qualify as candidates for strand pelletizing operations merely because the lands of the cutter teeth were too wide. As previously stated, the lands are the surfaces that are reconditioned when necessary to restore sharpness of the cutting edges of the cutter teeth. Each reconditioning procedure increases the width of the land. When the land width becomes great enough to adversely effect the cutter performance it becomes necessary to rework the tool flank and the root radius to restore the land to optimum width for beginning a new cycle of sharpenings. This costly operation is called regulleting and is periodically required in the regular maintenance of any multiple tooth circular cutter.
Thus, in order to achieve optimum performance in a strand pellitizng operation, it is necessary to determine how the raw material will react during cutting. The reaction of the raw material during cutting to form the pellets can be enhanced by optimizing three parameters which are present in all strand pelletizing machines. The parameters are: the profile of the cutter teeth and the bed knife; the gap between the cutter teeth and the bed knife; and the orientation of the feed plane to the cutter axis.
To the inventor's knowledge, an apparatus does not exist which enables manual simulation of the automatic cutting of strands as above disclosed. An apparatus is needed which can take into account the above parameters without the need of designing a prototype circular cutter. Thus, it is an object of the present invention to provide a manual cutting apparatus which simulates the action of an automatic circular cutter. The present invention enables multiple settings, adjustment, or the like of its cutting mechanism to simulate the above parameters and different material to provide data to enable optimum design of circular cutters. The present invention further provides the art with an apparatus which enables the design of rotary cutters without expending a substantial sum of money on designing prototype rotary cutters.
From the subsequent detailed description taken in conjunction with the attached drawings and appended claims, other objects and advantages of the present invention will become apparent to those skilled in the art.